The preparation of a phenol by an oxidative decarboxylation has been known for a long time. NL-B-90.684 already disclosed such a process, with oxidation, decarboxylation and hydrolysis all being carried out in one single process step, at a temperature of at least 200.degree. C. and preferably 230.degree.-250.degree. C.
This so-called Dow phenol process has been the subject of a number of patent publications, which aimed to eliminate the major drawback of the above process, i.e. the formation of a considerable number of byproducts, mainly in the form of tar.
NL-A-70.00685, for instance, discloses a two-step process for the preparation of a phenol from a benzene monocarboxylic acid. First, oxidation and decarboxylation are simultaneously effected at a temperature of 230.degree.-240.degree. C. The corresponding phenyl benzoate obtained is subsequently hydrolysed in the presence of oxygen at a temperature of about 200.degree. C.
NL-A-78.07199 describes a method for the preparation of a phenol on the basis of a three-step process. First an oxidation is carried out, in the absence of water, at a temperature of preferably 120.degree.-170.degree. C.; then a decarboxylation in the absence of oxygen and water, at a temperature of, preferably, lower than 220.degree. C., and the third step comprises hydrolysis of the aryl benzoate obtained, carried out in the absence of oxygen and preferably at a temperature of about 220.degree. C. According to the applicant of the above-mentioned patent, the presence of water in the decarboxylation is to be avoided, which is achieved by addition of a dehydrating agent, by an azeotropic distillation with an extra hydrocarbon added, or by stripping with a dry, inert gas.
With the aim of preventing tar formation it has been suggested to have the reaction take place in the gas phase on a solid copper-containing catalyst; see for instance NL-B-107561 and NL-B-110374. These, as well as later publications, which are mainly aimed at optimization of the catalyst (e.g. NL-A-7810528, EP-A-52839 and EP-A-40452), do not disclose systems whereby tar formation is actually suppressed, however. In all cases it appears that after some time the catalyst is covered with a black tar-like coat, which in particular affects its activity.
However, all the processes referred to have not been able to realize phenol formation at a high yield with little, if any, tar formation in an economically sound way.
According to NL-A-78.07199, for instance, a very large amount of catalyst is used, such that it takes the form of a separate, solid phase, rather than a homogeneous one, with all consequences thereof (including troublesome separation from the reaction mixture of the metal oxides formed). In addition, the methods employed to avoid the presence of water in the decarboxylation are rather expensive and laborious; the high yields cited cannot be reproduced.